1. Field of the Invention
The present invention pertains to the field of thin-film integrated circuit components, and, more specifically, to metal insulator semiconductors ("MIS") devices. Still more particularly, the MIS devices contain a ferroelectric material as the insulator.
2. Description of the Prior Art
Ferroelectric materials can be used as a gate insulator of metal oxide semiconductor field effect transistor ("MOSFET") devices. In this application, the spontaneous polarization serves to modulate the gate-channel conduction. The channel conduction state can, for example, be utilized as an indicator of a memory storage state. Silicon substrates are the most commonly used types of substrates for these applications. A problem exists because the ferroelectric polarization phenomenon is greatly reduced or even completely dissipated when the ferroelectric materials are deposited directly on the silicon surface. This reduction in ferroelectric polarization can be observed as the distance between positive and negative direction switching curves on a hysteresis plot of capacitance versus bias voltage. A very small separation between the positive and negative switching curves will indicate a correspondingly small ferroelectric polarization. Conversely, a large separation indicates a correspondingly large polarization.
Metal oxide ferroelectric materials have traditionally required firing or annealing at very high temperatures to provide a lattice structure that is capable of exhibiting ferroelectric polarization. These high firing temperatures induce diffusion of compounds between adjacent thin-film layers. One aspect of this diffusion is thought to be the production of a low dielectric layer that is formed between the ferroelectric material and the silicon substrate. This low dielectric layer acts as a parasitic capacitor and screens the applied electric field by producing a surface charge that reduces the ability of the applied electric field to reach the ferroelectric material. The screening effect reduces the magnitude of polarization obtainable from the ferroelectric material. The parasitic ferroelectric capacitor effect varies with the applied field and, consequently, the amount of screening or effective field-drop across the ferroelectric capacitor varies with the applied field.
Prior attempts at producing MIS devices using a ferroelectric material as the insulator have produced devices having capacitance versus voltage curves that present numerous problems. As indicated above, a primary problem is a reduction in the ferroelectric polarization phenomenon, which may be completely dissipated. Another problem is that the capacitance versus voltage curves rise at a low angle, not a steep one. This low angle indicates switching of the material over a very wide voltage range. The low-angle rise makes it possible to partially switch the polarization state of the ferroelectric material with cumulative effect due to noise. Electronic memories containing low-angle rise ferroelectric materials would, therefore, be subject to noise-induced read errors. Of course, the lack of any polarization at all would make it impossible to use the material in a non-destructive readout MOSFET ferroelectric memory where the polarization state controls the gate current.